1. Field of the Invention
This invention relates to a plated-through-hole method. More particularly, this invention relates to a method for depositing a thin layer of material such as metal onto the entire inside surface of a hole in an object.
2. Description of the Prior Art
Presently there exist many techniques used in the manufacture of a printed circuit board. Basically, the four different types of printed circuit boards are single sided boards, double sided boards, multi-layered boards, and flexible boards. Except for single sided boards and single sided flexible boards, it is necessary to provide electrical continuity via the holes in the boards, between the conductive pattern on each side of the board. This is accomplished by plating a layer of conductive material onto the inside surface of the hole in the board, and is referred to as the plated-through-hole operation of the manufacturing process. The only presently available plated-through-hole methods consist of electrolytic plating, electroless plating, and immersion plating. Each of these methods have their own particular advantages and disadvantages which largely depend on the type of printed circuit board being treated.
By way of example, electroless plating, which is probably the most widely used commercial method, basically comprises subjecting the boards to a cleaning procedure, an activation procedure, and then a copper deposition procedure. More particularly, the cleaning procedure includes cleaning the boards with an alkaline cleaner to remove light oil and grease, fingerprints, etc. The holes in the boards are then "conditioned" to accept activation and subsequent electroless copper deposition. The conditioning of the holes involves removing epoxy smear caused by the drilling operation. Drill smear can be removed from within the holes by utilizing an etchback process, or by utilizing my novel plasma desmearing apparatus and method which is disclosed in my U.S. Pat. No. 4,425,210, the disclosure of which is hereby incorporated by reference. After drill smear is removed from within the holes, the holes are preetched by using ammonium persulfate, sodium persulfate, or hydrogen peroxide/sulfuric acid. This removes copper oxide and etches the copper to a uniform matte finish. Next, the boards are dipped into sulfuric acid to remove any persulfate residues which would interfere with adhesion. The boards are then thoroughly rinsed in water to remove any sulfuric acid residues. This completes the cleaning procedure of the plated-thru-hole operation.
The activation procedure comprises subjecting the boards to a preactivator (pre-dip) and then an activator (catalyst) which seeds particles of tin/palladium onto the nonconductive holes in order to provide a basis for subsequent electroless copper deposition. After one or more water rinses, the boards are immersed in an accelerator which dissolves the tin from the tin/palladium complex that was deposited in the holes by the activator. The activation procedure is completed by thoroughly rinsing the boards with tap water or deionized water.
The copper deposition procedure comprises immersing the boards into an electroless copper bath which contains copper sulfate or chloride, sodium hydroxide, a reducing agent such as formaldehyde, and a complexing agent which holds the copper in solution. The boards remain in the copper bath for an appropriate length of time (depending on the desired thickness of the deposited layer of copper), and are removed and rinsed in water. After scrubbing and drying the boards, the boards are ready for the image transfer operation.
From the foregoing, it should be appreciated that electroless copper deposition is an extremely lengthy process. Moreover, the multitude of variables involved in the process makes the process more an art than a science, and results in nonuniform deposition of the copper within the holes of the printed circuit boards.
Therefore, it is an object of this invention to provide an apparatus and method which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the art of depositing metal on the inside surface of the holes in a printed circuit board.
Another object of this invention is to provide an apparatus and method for depositing a material onto an object by using plasma gas techniques.
Another object of this invention is to provide an apparatus and method for depositing a material on the surface of an object in a controlled process with a high level of precise repeatability and at cyclic rates substantially reducing the overall cost of the process.
Another object of this invention is to provide an apparatus and method for depositing the material uniformly on the surface of the object.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner by modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.